This invention relates, generally, to a solid state switch which utilizes a light-activated silicon controlled rectifier. More particularly, this invention relates to a solid state switch capable of reliably switching resistive, inductive, and capacitive loads.
In prior arts, the extreme sensitivity of the silicon controlled rectifier, in general, and light-activated silicon controlled rectifiers, in particular, to line noise and transients, limited their application. Any attempt to improve their performance under hostile conditions resulted in further limitations on the type of load that could be switched. Silicon controlled rectifiers are normally used to switch high currents to an A.C. load by means of controlling low currents through a PN junction of the silicon controlled rectifier, commonly called the "p-gate" or the "gate." Light-activated silicon controlled rectifiers differ from the previous description in that the gate current needed to trigger the silicon controlled rectifier is generated by an optically-coupled light emitting diode, thus enabling total isolation of the control voltage from the load voltage.
Both types of silicon controlled rectifiers inherently suffer from susceptibility to false switching due to the presence of transient noise on the A.C. line or electromagnetic interference (EMI) in the vicinity of the silicon controlled rectifier. This false switching phenomenon will deliver current to loads when no such current is called for. Many attempts have been made in the past by both the makers and users of silicon controlled rectifiers to minimize the effect of these transients on switching the silicon controlled rectifiers. The makers have used various techniques, one of which is a partial internal short between the gate and the cathode of the silicon controlled rectifier. The users revert to various circuit techniques that tend to clamp the gate of the silicon controlled rectifier to a voltage near ground (ground is considered here to be the cathode potential) or even to a negative potential. One serious drawback to these circuits is that they are applicable to use with resistive loads, but are not applicable for use with inductive or capacitive loads where a phase shift between the current and voltage is present.